Diluted magnetic semiconductors (DMS) are exciting materials not only
because of possible applications in spintronics but also from a
basic-physics point of view. One main aspect is that the magnetic
interactions between impurity spins and thus ferromagnetic order can be
controlled by changing the carrier concentration by doping or an applied
gate voltage---a possibility that does not exist in ferromagnetic metals.
Also, magnetic semiconductors add magnetic ordering and its relation to
positional disorder to the long list of phenomena that can be studied in
semiconductors. An overview over key properties of DMS is given in this
talk, focussing on important experiments and the resulting constraints for
a theoretical description. Important aspects are strong disorder effects
and spin-orbit coupling. Relations to quantum order at low temperatures are
also pointed out.